Trans RINA, Vol 154, Part C1, Intl J Marine Design, Jan - Jun 2012 2.3 (b) Motions in Waves and Comfort Measures
The vessel motions due to waves were predicted using Seakeeper – a linear strip theory method in the style of Salvesen et al. [6]. Two scenarios were considered (details are given in Table 3):
vessel at anchor or in a marina in a very slight sea-state – the so-called
“Party” condition (mooring forces were neglected); and
vessel underway at a cruising speed of 16kts in a higher sea-state, as might be encountered when travelling
locations – the “Cruise” condition.
The motion sickness incidence (MSI) after two hours exposure
was computed at different longitudinal
positions along the length of the vessel (Figure 3). That is the percentage of people who can be expected to vomit after having been subjected to the motions for a period of two hours [7, 8]. (The MSI value calculated should be considered a guide for comparison of
the relative
performance of different vessels only, rather than a true absolute measure of sickness incidence. This is because sickness can be extremely sensitive to other gender, age, habituation and other stimuli
factors: [9].) The
performance measure extracted from the analysis was simply the minimum MSI along the length of the vessel for each of the two scenarios considered; assuming that the vessel layout could be adjusted so that MSI-critical systems (e.g. the bar) could be sited accordingly.
Table 3: Two scenarios considered for the sea-keeping calculations.
Vessel speed [kts]
Characteristic wave height [m] Modal period [s] Wave heading
Wave spectrum type
“Party” 0.0 0.5 2.0
“Cruise” 16.0 2.0 7.1
Head seas
1-Parameter Bretschneider
between two such “Party”
Section 11.2.1.1 .1a
Table 4: Stability criteria considered. Description
Area under GZ curve from 0 to 30 deg. heel shall not be less than
.1b Area under GZ curve from 0 to 40 deg. heel shall not be less than
.2 Area under GZ curve 30 to 40 deg. heel shall not be less than
.3 Maximum GZ at 30 deg. or greater heel shall not be less than
.4 Angle at which maximum GZ occurs shall not be less than
.5
Initial metacentric height (GMt) shall not be less than
Required value
0.055 m.rad 0.090 m.rad
0.030 m.rad 0.2 m
25 deg. 0.15 m
2.3 (c) Hydrostatic Stability Criteria
Virtually all vessels must comply with hydrostatic stability criteria specified by class. A small subset of intact-vessel stability criteria, which are typically applied to this class of vessel, were selected from the Large Commercial Yacht Code intact stability standards for monohull vessels, section 11.2.1.1 [10]. These criteria are summarized in Table 4.
Figure 4: Effect of displacement on maximum VCG for three different design variants.
Figure 3: Typical MSI distribution over the length of the vessel.
C-20
In order to obtain a meaningful performance measure of stability, the maximum vertical centre of gravity (VCG) at which all criteria were just passed was calculated for a range of displacements using Hydromax. A typical curve of maximum allowable VCG against displacement, for three representative design variants, is shown in Figure 4. The measure of performance used was the area under the maximum allowable VCG curve integrated over the displacement range of 1800t to 2600t. This measure was chosen because early in the design process, neither the VCG nor the displacement would be known with
©2012: The Royal Institution of Naval Architects
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64